A knife is not simply a cutting instrument. There are dozens of types of knives, from Arkansas toothpicks to X-Actos. Keeping track of them all and their legality is not easy since knives are regulated by a jumble of federal, state, and local laws, much to the chagrin of knife-rights advocates. A quick guide to some of the most common—and commonly confused—blades out there.
Assisted-opening knives: These are folding knives that are opened by putting pressure on the blade until a spring-loaded mechanism flips it out the rest of the way. These knives, which can be opened with one hand, are the most popular nonkitchen knives in America, according to the American Knife and Tool Institute, the knife industry's lobbying group. They are not switchblades, which are largely illegal, though this distinction is not always recognized by law. (The Federal Switchblade Act was amended in 2009 to exempt one-handed openers.) This video explains the difference between the two kinds of knives.
Switchblade: A knife with a spring-loaded blade that is opened by pressing a button or switch. Also known as flick knives or automatic knives, their importation and interstate transport are prohibited under the 1958 Federal Switchblade Act. (The law does provide an exemption for members of the military, law enforcement, and people with one arm.) Most famous as the weapon of choice of well-groomed '50s hoodlums in movies like West Side Story and Rebel Without a Cause.
Daggers: There are many types of daggers, which is a general term for a pointed knife intended for stabbing. Variations include the dirk, the stiletto (including some switchblades), the butterfly knife (see below), and the Arkansas toothpick. Daggers are banned in several states, though the AKTI complains that the legal definitions are overly vague and include knives that "can be a useful tool for innumerable lawful and especially defensive purposes."
Gravity knife: A knife that opens when a latch is released and the knife is turned so the blade slides or "falls" out. The best known models were carried by World War II German paratroopers, who needed knives that could be easily opened with one hand in case their chutes got stuck in a tree. Gravity knives are banned by the Federal Switchblade Act.
Ballistic knife: A knife with a spring-loaded blade that can be "fired" from the handle like a missile. Currently illegal under federal law, they are mostly found in virtual combat. See one in action below (and check out this photo of one being demo'd for former Sen. Al D'Amato, a ballistic-knife foe).
Bowie knife: Webster's defines a bowie as "a stout single-edged hunting knife with part of the back edge curved concavely to a point and sharpened." However, the AKTI maintains that the term "cannot be satisfactorily defined with sufficient precision." Either way, it's a big-ass blade based on the butcher knife that 19th-century slaver and Alamo defender James Bowie wielded to become "the South's most formidable knife fighter." Or, as one Bowie-toting Aussie famously put it, "That's a knife!"
Butterfly knife: Also known as a balisong, this is a folding dagger whose blade is kept between two rotating handles. They are illegal in many places and popular among those who can spare a digit or two, including humanitarian/weapons buff Angelina Jolie.